1. Field of the Invention
This invention relates to vehicle-mounted electrical generating systems, and more particularly to apparatus incorporated with the electrical generating system of a vehicle for producing the electrical power necessary for the electrical system of the vehicle and also, at the same time, producing power for operating auxiliary power loads such as electrically-driven power hand tools, lamps, and the like.
2. Description of the Related Art
Portable electric tools, lamps, and like electrically-powered accessories must sometimes be used at remote locations where the usual household 110-volt alternating current (AC) electrical supply is unavailable. In such instances, a vehicle-mounted electrical generating system can provide an electrical supply.
Vehicle-mounted electrical generating systems normally include an engine-driven alternator. These automotive engine-driven alternators are used to produce low-voltage AC electrical power which is then rectified to produce direct current (DC) electrical power for use by the vehicle's electrical system. Thus, these alternators provide a readily available potential source of power for portable tools, lamps, and like electrically-powered accessories.
The magnitude of the electromotive force generated in alternator stator windings is determined both by the strength of the magnetic field that is linking with the alternator windings and by the rate of rotation of the magnetic field, which changes the flux linkages between the magnetic field and the alternator stator windings.
The electric windings in the stator are designed so that the alternator is capable of generating a voltage sufficient to charge the vehicle battery when the alternator rotor, containing the electromagnetic field, is rotated at a low, or idling, engine speed. When the alternator rotor is driven at a higher speed, the alternator is capable of generating a higher voltage than is required to charge the vehicle battery and to supply the vehicle electrical system. This higher voltage is normally not desirable because it would harm the battery and the vehicle electrical system. Therefore, when the alternator rotor is driven at a higher speed, the alternator voltage regulator acts to reduce the current in the alternator field coil and thus to maintain the generated voltage at the desired level for charging the vehicle battery and for the vehicle electrical system.
The typical vehicle-mounted automotive alternator can produce only a limited amount of electric current, even at short- circuit conditions. The amount of electric current that the automotive alternator can generate is determined by the physical size and shape of the alternator and by the design of the electric windings in the alternator stator. An exemplary conventional 80 amp alternator can generate a maximum of only 80 amps, and can do so only at very high engine speeds. This exemplary 80 amp alternator can generate only 55 amps continuous-duty without overheating. With the maximum generated continuous-duty electric current limited to 55 amps by the physical construction and design of the vehicle alternator, and with the maximum generated voltage limited to 14 volts DC by the action of the voltage regulator to protect the vehicle battery and electrical system, the maximum continuous-duty generated output power of the exemplary alternator is limited to approximately 770 watts.
Alternators ordinarily found in the vehicle-mounted electrical generating systems for automobiles and light trucks are typically capable of producing a maximum of 600 to 800 watts of continuous duty electrical power at moderate engine speeds without overheating. Converters are available that take the low-voltage AC generated by the alternator to produce high-voltage DC power, on the order of 100-110 volts, that can be used to power universal-motor tools, lamps, and other accessories compatible with DC power. These converters provide a source of electrical power at remote locations or wherever 110-volt AC power is unavailable.
Unfortunately, many of the most useful power tools and accessories that would be used at remote locations require a minimum of 1200 to 1400 watts of electrical power. These include drills, winches, and lamps. Most of the presently available converters can alter both the voltage and the form (AC or DC) of the electrical power from the vehicle alternator, but cannot increase the power that the typical automotive or light truck alternator is capable of producing, which is 600 to 800 watts of continuous duty power at moderate engine speeds.
Some presently available converters disconnect the vehicle-mounted alternator stator windings and rotor field windings from the vehicle electrical system and then separately excite the alternator field windings to produce high voltage while the alternator is operated at a relatively high speed. These systems are relatively expensive because of the need for a separate source for the field current while the alternator and converter are in the auxiliary power mode of operation. In addition, these converters cannot be used for any significant length of time without risking completely discharging the vehicle battery, which is without a source of recharging current while the converter is in use.
Other converter systems are available for use with special heavy-duty alternators that are capable of producing 1200 to 1400 watts of electrical power, but these heavy-duty alternators are generally limited to heavy equipment and vehicles of rugged construction, and are prohibitively expensive.
Ranchers, farmers, construction workers, and others who could greatly benefit from a readily available, inexpensive, and reliable source of power when away from 110-volt AC household power are thus hard pressed to find a suitable solution.
Thus, there is a need for an inexpensive power converter system that will allow the 600 to 800 watt alternator normally found in the vehicle-mounted electrical generating systems for automobiles and light trucks to produce, at moderate engine speeds, 1200 to 1400 watts of continuous duty electrical power for use as an auxiliary power source for power tools, lamps, and other accessories while still, at the same time, providing the correct DC electrical power for the vehicle's electrical system.